After being part of the design and construction process for several duplex projects, we’ve learnt a thing or two on what makes a better site for a new attached or detached dual occupancy. Here are a few we can share with you:

First & foremost, the site should be in a suburb within a Local Government Area that allows detached or attached dual occupancy to be built and subdivided.
Corner lots and very wide (20m+) lots have a larger frontage, and can potentially be subdivided after completion with 2 new detached dwellings instead of 2 attached dwellings that share a party wall.
Wider lots have the benefit over narrow lots by having the ability to fit more width ways, such as 2 rooms and a hallway/corridor, and more width in the front yard which will result in more external car spaces.
Narrow lots have the benefit over wide lots by rooms on the first floor having better access to windows, and a hallway being beside the party wall on the first floor, creating a buffer. Rear yards will also seem larger and more usable since the area will be longer.
The most suitable sites have a gentle slope towards the street for drainage. Sites that slope towards the rear and are surrounded by other lots will likely require an easement for drainage and sites that are too flat may have issues with drainage and flooding. There are also suitable sites that have substantial slope towards the street, these sites can accommodate basements.
A site with a north facing rear will provide both dwellings with pleasant sunlight during the day to all its living areas at the rear of the duplex in comparison to a site with north facing the front elevation or north to one side of the duplex.

In conclusion, the most suitable sites are within a LGA that allows dual occupancies, wide or on a corner lot, have a gentle slope towards the street (unless you’d like a basement), and north to rear facing orientation.